Magnetic random access memory (MRAM) is a nonvolatile memory technology that uses magnetization to represent stored data. An MRAM generally includes a plurality of magnetic cells in an array. Each cell represents one bit of data. A cell includes a magnetic element, such as an MTJ device.
Ferromagnetic plates of an MTJ device typically include a free layer and a pinned layer (fixed layer) separated by a thin tunneling barrier layer. The plates are associated with a magnetization direction (or orientation of magnetic moments). In the free layer, the magnetization direction is free to rotate. An anti-ferromagnetic layer may be used to fix the magnetization of the pinned layer in a particular direction. A bit is written to the MTJ device by changing the magnetization direction of a free layer of the ferromagnetic plates of the magnetic element. Depending upon the orientations of the magnetic moments of the free layer and the pinned layer, the resistance of the MTJ device is changed.
Spin torque transfer (STT) is one technique used for writing to MTJ devices. Where a spin-polarized current is applied to the free layer, electrons are repolarized. The repolarizing of the electrons leads to the free layer experiencing a torque associated with the change in the angular momentum of the repolarized electrons. As a result, if the current density, or switching current, is high enough, this torque can switch the direction of the magnetization of the free layer.
The advantages of using STT for writing to magnetic elements include smaller bit cell size and nonvolatile storage. Such advances have led to more demand to improve performance and promote scalability for broader application. For example, there is a need to design MRAMs that reduce magnetic field interference between neighboring MTJ devices. In addition, there is a need to reduce switching currents.